loop-util: make loop_device_make() return fd in all code paths

loop-util: make loop_device_make() return fd in all code paths

Previously, loop_device_make() would return the device fd in one success
code path, but not the other (where' we'd just return 0).
loop_device_open() returns it in all cases.

Hence, let's clean this up, and make sure in all success code paths of
both functions we return it (even though it strictly speaking is
redundant, since we return it in LoopDevice anyway, and currently noone
actually relies on this).